Generally, a superconducting power apparatus which is operated at ultra-low temperatures is essentially composed of a current lead for supplying large current at from room temperature (300K) to an ultra-low temperature (77K).
Specifically, FIG. 1 shows the structure of a general superconducting power apparatus. As shown in the drawing, the superconducting power apparatus received in an ultra-low temperature container 1 is connected with an ultra-low temperature freezer 3 for ultra-low temperature cooling. Further, in order to supply current to the superconducting power apparatus from the outside, a normal conducting current lead 4 connected to an external power supplier is provided in the temperature range from 300 K to 77 K, and also, in the ultra-low temperature range below 77K, a superconducting current lead 5 which is connected to the superconducting power apparatus 2 is provided.
The normal conducting current lead 4 is designed to generate Joule heat in a predetermined amount and to have minimum heat conductivity, such that the normal conducting current lead 4 is prevented from being cooled due to the superconducting current lead 5 and heat penetration from the outside is minimized.
To this end, the shape of the normal conducting current lead 4 is determined to optimally generate Joule heat at maximum rated current and to minimize the heat conductivity.
That is, when the material for the current lead is determined according to being that which provides for the optimal generation of Joule heat and the minimization of heat conductivity, applied current (I), the length (L) of the current lead, and the cross-sectional area (A) of the current lead are determined according to the relationship of I×L/A=C (constant), in which C is the value which is determined depending on the type of material.
Recently, as the capacity of the superconducting power apparatus has increased, the demand for application of large current is increasing. In order to comply therewith, there is a need for a current lead having a large cross-section so as to enable the application of a large amount of current.
As seen with reference to the above relationship, when the current (I) is increased at a predetermined length (L), the cross-sectional area (A) must be increased.
However, as known in the art, when current (alternating current) flows through a conductor, it is concentrated on the surface of the conductor. That is, the current density is increased as close to the surface of the conductor.
A drawing for explaining such a phenomenon is depicted in FIG. 2. FIG. 2 is a perspective view showing a conventional normal conducting current lead and a graph showing the distribution of current flowing through the cross-section thereof.
As shown in FIG. 2, the current density is exponentially increased in proportion to the increase in the radial distance from the center of the conductor or in proceeding toward the outer surface of the conductor.
Due to such a phenomenon, the cross-sectional area of the current lead should be large so as to transmit large current, and accordingly, the volume and weight of the current lead are increased.
Further, as mentioned above, a current lead having at least a predetermined length should be ensured to minimize the heat penetration. So, it is not easy to reduce the weight of the current lead.
With the aim of solving such problems, the current lead may be manufactured in the form of a tube. If so, the weight of the current lead may be reduced but the diameter thereof should be maintained as it is, thus making it impossible to reduce the size of the current lead.
Hence, the current lead having a large cross-sectional area must be used, but a difficulty comes about in terms of a manufacturing process, and also the size of the superconducting power apparatus is increased, making it difficult to reduce the total size of the system.